v3 branch is not working for .net core paltform

[yeibl]

i fix bug that is no working for .net core platform , i found it in BackwardCompatibility class .
when reflect loggingEvent class and get GetProperties like this
`
PropertiesDictionary compositeProperties = loggingEvent.GetProperties();
if (compositeProperties != null && compositeProperties.Count > 0)
{
var properties = new BsonDocument();
foreach (DictionaryEntry entry in compositeProperties)
{
properties.Add(entry.Key.ToString(), entry.Value);
}

			toReturn.Add("properties", properties);
		}

change to
PropertiesDictionary compositeProperties = loggingEvent.GetProperties();
if (compositeProperties != null && compositeProperties.Count > 0)
{
var properties = new BsonDocument();
foreach (DictionaryEntry entry in compositeProperties)
{
properties.Add(entry.Key.ToString(), entry.Value.ToString() ?? "");
}

			toReturn.Add("properties", properties);
		}

`
then is work for me

[gimmi]

Hi yebil,

thanks for taking time to look into it. Can you please submit a PR, se we can more easily evaluate it?